1. Field of the Invention
The present invention relates to a fixing device of an image forming apparatus. More particularly, the present invention relates to a fixing device that includes an endless member that is brought into pressure contact with a fixing member.
2. Description of the Related Art
A heat fixing device is used in an image forming apparatus such as a copier, a printer, or a facsimile. A typical heat fixing device fixes an unfixed toner image onto a paper sheet by use of heat and pressure. For example, Japanese Patent No. 3298354 discloses a conventional heat fixing device that employs a belt-nip system. In such a belt-nip system, a fixing member, such as a fixing roller, is brought into pressure contact with an endless pressure-contact member, such as an endless belt, to form a nip portion between the fixing member and the endless pressure-contact member. A paper sheet with an unfixed toner image thereon is passed through the nip portion. As a result, the paper sheet is heated and pressurized, so that the unfixed toner image is fixed onto the paper sheet.
The conventional heat-fixing device includes a heating-fixing roller having an elastically deformable surface that is subjected to local elastic deformation at the exit (nip exit) of a nip portion. Such local deformation at the nip exit, however, leads to variations in the surface speed of the heating-fixing roller (see FIG. 4 of Japanese Patent No. 3298354). The variations in the surface speed of the heating-fixing roller leads to shifting of writing position of an image, which in turn leads to degradation in image quality.
In addition, a hard member is pressed against an elastic layer of the heating-fixing roller to locally cause deformation of the elastic layer, so that a nip portion having a small curvature is formed. This configuration, however, imposes a large load on the paper sheet, which disadvantageously leads to damage to the recoding medium and/or curling of the recoding medium.